1. Field of the Invention
This invention relates to a method and apparatus for producing an extrudate from a thermoplastic polymer material.
2. Description of Prior Art
The invention is particularly concerned with the continuous extrusion of a polymer material to form an extrudate about a continuously traveling conductor wire to form an insulated wire. Such insulated wires are widely used in electric power cables and communication cables.
In producing such conductor wires it is important that the extrudate be uniform in its characteristics. In order to achieve this it is important that the polymer material be melted and worked to a uniform state. On the other hand, it is desirable that the extrusion proceed at a high rate to achieve an efficient line speed. In general the rate of extrusion dictates the line speed in cable production, the extrusion rate being dictated by the aforementioned need for a uniform extrudate.
Thus, on the one hand, it is desirable to retain the polymer material in the extruder for a long period to ensure that the polymer material is fully melted and worked throughout to produce a uniform extrudate; and, on the other hand, it is desirable to pass the polymer material through the extruder in the shortest time possible to achieve a highline speed.
It is well known that in melting a polymer material in a screw extruder, the melt proceeds non-uniformly. The polymer material is subjected to heat from the heating elements of the screw extruder as well as heat developed as frictional heat by the different forces to which the polymer material is subjected including the shear forces developed between adjacent particles of solid or semi-solid polymer material, between adjacent portions of molten polymer material, the interaction of the advancing screw thread and the polymer material in its different physical states, and the interaction between polymer material and the interior wall of the barrel of the extruder as the advance of the polymer material is opposed by the barrel wall.
Organic polymer materials are poor conductors of heat and this also favours non-uniform heating so that in any particular length of advancing polymer material in a screw extruder, the polymer material throughout a cross-section of such length, will likely occur in different physical states ranging from the solid state to the fully fluid molten state.
A further problem is that the physical properties of polymer materials, particularly polyolefins may change significantly during extrusion. In particular the melt flow or melt index may deteriorate significantly during extrusion so that the polymer flows to readily. This deterioration results in part from shear degradation and in part from heat degradation. Employing the Normal ASTM Melt Index Test, the melt flow or melt index may increase by 80 to 90%. In other words the melt index of the polymer extrudate will be 80 to 90% higher than the melt index of the polymer prior to extruding. This increase in the melt index is generally unacceptable, in particular the Rural Electrification Administration (REA) of the U.S.A. stipulates that the melt index of the polymer extrudate, of insulated wires, not be greater than 50% of the melt index of the polymer prior to extrusion. This requirement has been widely adopted around the world.
In general as the melt index parameter increases the resistance to stress cracking or ESCR (environmental stress cracking resistance) decreases, which means that the insulated wire is more susceptible to crack formation in the insulation, and the problems associated therewith.
Attempts have been made to have the REA increase the maximum permitted change to 110%, which would be considerably less exacting requirement, but so far no change has been made so that insulated conductors must continue to meet the exacting requirement of a not more than 50% change in the melt index.